Bjerrum(4.15)has reported on the effects of time on the shaft friction of piles driven into
soft clays. He observed that if a pile is subjected to a sustained load over a long period the
shearing stress in the clay next to the pile is carried partly in effective friction and partly in
effective cohesion. This results in a downward creep of the pile until such time as the
frictional resistance of the clay is mobilized to a degree sufficient to carry the full shearing
stress. If insufficient frictional resistance is available the pile will continue to creep down-
wards. However, the effect of long-period loading is to increase the effective shaft resistance
as a result of the consolidation of the clay. It must therefore be expected that if a pile has an
adequate safety factor as shown by a conventional short-term loading test, the effect of the
permanent (i.e. long-term) working load will be to increase the safety factor with time.
However, Bjerrum further noted that if the load was applied at a very slow rate there was a
considerable reduction in the resistance that could be mobilized. He reported a reduction of
50% in the adhesion provided by a soft clay in Mexico City when the loading rate was
reduced from 10 to 0.001 mm per minute, and a similar reduction in soft clay in Gothenburg
resulting from a reduction in loading rate from 1 to 0.001 mm per minute. These effects must
be taken into account in assessing the required safety factor if a pile is required to mobilize
a substantial proportion of the working load in shaft friction in a soft clay.
No conclusive observations have been published on the effects of sustained loading on
piles driven in stiff clays, but there may be a reduction in resistance with time. Surface water
can enter the gap and radial cracks around the upper part of the pile caused by the entry of
displacement piles, and this results in a general softening of the soil in the fissure system
surrounding the pile. The migration of water from the setting and hardening concrete into
the clay surrounding a bored pile is again a slow process but there is some evidence of a
reverse movement from the soil into the hardened concrete(4.16). Some collected data on
reductions in resistance with time for loading tests made at a rapid rate of application on
piles in stiff clays are as follows:
Type of pile Type of clay Change in resistance Reference
Driven precast London Decrease of 10% to 20% at 9 months Meyerhof and
concrete over first test at 1 month Murdock(4.9)
Driven precast Aarhus Decrease of 10% to 20% at 3 months Ballisager(4.17)
concrete (Septarian) over first test at 1 month
Driven steel tube London Decrease of 4% to 25% at 1 year over Tomlinson(4.2)
first test at 1 month
It is important to note that the same pile was tested twice to give the reduction shown
above. Loading tests on stiff clays often yield load/settlement curves of the shape shown
in Figure 11.13b (Section 11.4.2). Thus the second test made after a time interval may
merely reflect the lower ‘long-strain’shaft friction which has not recovered to the original
peak value at the time of the second test. From the above data it is concluded that the fairly
small changes in pile resistance for periods of up to one year after equalization of pore
pressure changes caused by installation are of little significance compared with other
uncertain effects. An increase should be allowed only in the case of soft clays sensitive to
remoulding.
164 Resistance of piles to compressive loads